Vinyl acetate is an ethylenically unsaturated monomer used for the production of various polyvinyl resins. It is stabilized against premature polymerization by inhibitors such as hydroquinone and diphenylamine. Such inhibitors are routinely added to vinyl acetate after production, maintained in the vinyl acetate during storage, shipping and the like, and removed prior to, or overcome during, polymerization of vinyl acetate to produce the desired resin.
Vinyl acetate is commonly produced commercially by vapor-phase reaction of ethylene and acetic acid with a palladium-based catalyst, such as a potassium or sodium acetate supported palladium catalyst system. For instance, a vapor-phase mixture of ethylene, acetic acid and oxygen is formed by passing ethylene through an evaporator containing acetic acid, and then it is passed over a fixed-bed catalyst, at elevated pressure and temperature, exiting therefrom as a product mixture. Such product mixture is typically comprised of the vinyl acetate product, ethylene, acetic acid, oxygen, and by-products/impurities. The vapor-phase product mixture is cooled by a heat exchanger(s), resulting in a vapor-liquid stream. Of the product mixture constituents, the acetic acid raw material and the vinyl acetate product have relatively high boiling points, and thus comprise the major portion of the liquid of the vapor-liquid stream, while the ethylene, oxygen, and carbon dioxide comprise the major portion of the vapor of the vapor-liquid stream.
Such vapor-liquid stream is fractionated and purified through a series of columns and washes to recover a product-quality vinyl acetate fraction and unreacted raw materials for recycling. In more detail, the vapor-liquid stream is fractionated in a column in which vinyl acetate and unreacted acetic acid (the crude vinyl acetate product) are collected as liquids at the bottom, while the vapor phase, the "light ends" or "overhead", are collected at the top. As noted above, the major portion of the vapor, and thus the light ends stream, is ethylene, oxygen and carbon dioxide. Some small amount of acetic acid may go out the top during this fractionation. If it does, then some vinyl acetate will also go out the top because vinyl acetate has a lower boiling point than acetic acid. The presence of acetic acid in the light ends stream is itself of little concern, but optimization of production requires the optimization of vinyl acetate collected at the bottom. Therefore production facilities and parameters are designed to minimize the amount of vinyl acetate in the light ends, and therefore the amount of acetic acid therein.
The light ends stream contains unreacted ethylene for recycling back to the evaporator. The light ends stream is processed through a series of wash columns to remove various by-products and then is sent to a compressor(s) to prepare the unreacted materials for reentry to the evaporator. The light ends stream is at least partially liquified by the compressor, which is referred to at times herein as the "light ends stream compressor".
Both vinyl acetate and ethylene, one of its raw material precursors, are unsaturated monomers capable of undergoing addition polymerization through their carbon-carbon double bonds. Addition polymerization may be initiated by free radicals or ions. Free radicals can be generated by the action of ultraviolet light, X-rays, heat, peroxides, oxygen, diazo compounds, and other means. Once initiated, an addition polymerization normally propagates itself by a chain-reaction mechanism, whereby polymer molecules comprised of thousands of monomer ("mer") units may be formed in a few seconds or less. The polymerization chain-reaction for a given polymer molecule is terminated when its growing end, which is itself a free radical, reacts and combines with another free radical, rather than a monomer molecule. Termination may also occur when a labile hydrogen is abstracted.
The vinyl acetate monomer is, like many ethylenically-unsaturated monomers, a volatile liquid at ambient room temperature and atmospheric pressure, and has a melting point of about -93.degree. C. The ethylene raw material is a vapor at ambient room temperature and atmospheric pressure, and has a melting point of -169.degree. C. Polymerization products of either generally would be solids at ambient room temperature and atmospheric pressure.
Formation of polymeric material in a light ends stream compressor will foul the compressor. Compressor fouling reduces compressor efficiency, and thus its production rate. Compressor fouling eventually requires that the compressor be shut down for cleaning, typically requiring the entire vinyl acetate production unit to be shut down, at least for some time period. Thus compressor fouling inevitably leads to lost production and increased maintenance costs. Fouling is caused by the presence of solid materials that coat the compressor blades and/or casing. Although solid material foulants may, to some extent, be introduced into the compressor by other means, analysis of such foulants has in the past revealed that they are comprised mostly of organic polymeric material. The major source of such foulants in such a compressor hence is the formation of polymeric materials from the unsaturated constituents of the light ends stream.
It is an object of the present invention to provide a method of inhibiting or reducing the fouling of a light ends stream compressor. It is an object of the present invention to provide a method for treating the light ends stream or compressor feed so as to retard the loss of compressor efficiency with run time by inhibiting or reducing the fouling of the light ends stream compressor. It is an object of the present invention to provide a method whereby the run time of a light ends stream compressor before clean-out is required is increased by inhibiting or reducing the fouling of the compressor. These and other objects of the invention are described in more detail below.